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(1) TNSPING was very slow from client i.e. from my machine as well as from server. It was taking lot of time to give output.
(2) lsnrctl status command was also very slow ( almost in hang state ) both from server as well as from client .
(3) Users were complaining slow query performance but were able to connect quickly though SAP ( Application ).

Diagnosis :

(1) Checked TNSPING response from both client as well as from server :

Server :
—————————————————————–
bash-3.2$ tnsping bip

TNS Ping Utility for Linux: Version 11.2.0.1.0 – Production on 28-MAR-2014 10:04:39

– It was all good. i was able to ping the IP & Hostname both from server as well as client without any delay or RTO.’

(9) Checked for /etc/hosts file whether the hostname was resolved in /etc/hosts file.

– It was resolved in host file.

(10) Check for Size of listener.ora file

– If the file exceeds to GB in linux X64. The listener performance will be slow. In my case it was not so as the size was in MB’s.

Solution :

After analyzing all the above information, it was clear that the issue was not related to oracle as the listener worked fine prior to today and there was no configuration changes made recently. There was no issue related to network.
I found that from oracle11g, oracle first resolves hostname using DNS entry & reads /etc/resolv.conf file first instead of /etc/hosts.

I found that there was a wrong DNS IP mentioned in /etc/resolve.conf file which we were not using. After commenting the same, it worked fine and the response time was dropped from couple of mins to 0sec on server.